SALIVA AND ITS PROSTHODONTIC IMPLICATIONS

INTRODUCTION

The secretions of the major and minor salivary glands,

together with gingival crevicular fluid constitute oral fluid which

provides chemical milieu of the teeth and oral soft tissue.

The environment of the oral cavity is to a large degree

created and regulated by saliva. The fact that the teeth are in

constant contact with and bathed by the saliva would suggest that

this environmental agent could profoundly influence the state of

oral health of a person. Saliva has manifold functions in protecting

the integrity of the oral mucosa. Most often, the dentist is called

upon to treat patients in whom he finds out a few of the symptoms

and clinical signs of abnormal salivary secretions and function.

Hence the knowledge of salivary glands, secretion and function is

important in diagnosis, treatment planning, treatment and in

predicting the prognosis.

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Sources of saliva

Saliva is a complex mixture of fluids with contributions from

the major salivary glands (parotid, submandibular and sublingual),

the minor / accessory glands and the crevicular fluid. Additionally

it contains a high population of bacteria normally resident in the

mouth, desquamated epithelial cells and transient residues of food

or drink following their ingestion. When referring to the fluid

normally present in the mouth the term whole saliva is commonly

used, as distinct from ‘direct saliva’ which is that flowing from

individual glands.

Formation of saliva

Saliva is formed in two stages: a primary secretion occurs in

the acini, then modified as it passes through the ducts. The primary

secretion is formed actively by the movement of sodium and

chloride ions into the lumen, creating an osmotic gradient which

leads to the passive movement of water. Other acinar components

are added here before the fluid enters the ducts, where sodium ions

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are actively reabsorbed (chloride ions follow passively to maintain

electrical equilibrium) and potassium and bicarbonate ions

secreted.

The macromolecular components (amylase mucous

glycoproteins etc.) are formed in the usual way in the acinar cell

endoplasmic reticulum, processed into secretory vesicles in the

golgi apparatus and are exported from the cell by exocytosis.

Signal transduction

When the nerve to the salivary gland is stimulated, the

transduction of this signal to increase the formation of saliva is

first brought about by the release of a neurotransmitter substance.

These include noradrenaline and acetyl holine, substance P and

vasointestinal polypeptide (parasymphathetics). When the

neurotransmitter arises at a secretory cell membrane it binds to and

activates a receptor (which may be stimulatory or inhibitory) on the

external surface of the membrane. This activates an intermediate

quanine nucleotide-dependent membrane protein known as ‘G’

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protein which in turn activates a regulatory enzyme on the inner

cytoplasmic surface of the cell. The regulator enzyme may be a

phospholipase C or adenyl cyclase. Phospholipase C is activated on

binding of acetyl choline at mucoarinic receptors and it controls the

intracellular pathway leading to the secretion of water and

electrolytes. Adrenyl cyclase is activated when noradrenaline binds

to -adrenergic acinar receptors. Activation leads to cexocylosis of

secretory proteins.

Control of Salivation

The salivary glands are unusual among the glands of the

digestive tract in being under purely nervous control. Hormonal

influences can alter the composition of saliva, but are not

responsible for its secretion.

The nerve supply to the major glands is derived from both

sympathetic and parasympathetic branches of the autonomous

nervous system. The sympathetic supply is trophic, the stimulation

of which leads to the release of secretory proteins such as amylase

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and vasoconstrictor. The parasympathetic nerves are secretomotor

and vasodilator. The nerves innervate the acinar cells, the direct

cells, blood vessels and myoepithelial cells.

Resting flow

Under resting conditions, without the exogenous stimulation

associated with feeding, there is slow flow of saliva which keeps

the mouth moist and lubricates the mucous membranes. This

unstimulated flow, which is present, the majority of the time, is

very important for the health and well being of the oral cavity. The

unstimulated flow is influenced by:

a) Circadian variation:

Unstimulated flow peaks at approximately 5pm in most

individuals with a minimum flow during the night. This variation is

independent of eating and sleeping behaviour.

b) Light and arousal:

If one is blind folded, or in an unlit room, the unstimulated

flow rate falls. This is probably associated with the effect of usual

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input in maintaining a state of arousal. Saliva flow in much reduced

during sleep.

c) Dehydration:

A loss of 8% body water results in a cessation of saliva flow.

The resultant drying of the oral cavity is a feature of thirst,

although thirst and water intake are under hypothalamic control and

not dependent upon oral dryness.

d) Exercise and stress:

A dry mouth is a feature of the ‘fright and flight’ response.

This is probably not a direct action of the sympathetic supply to the

gland, but the rather is due to inhibitory influences on the salivary

nuclei arising from the hypothalamus.

PSYCHIC FLOW

A mouth watering sensation is a universal experience on the

anticipation or sight of food, especially if temptingly presented

when hungry. However, although the sensation is of a sudden flow

of saliva into the mouth, it has not proved possible to demonstrate a

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large increase in flow rate in man arising from such ‘psychic’

stimuli.

Unconditional reflexes

The most important stimuli to salivation are those associated

with feeding; masticatory movement and especially taste.

Mastication

This is a reflex response wherein receptors in the muscles of

mastication, temporomandibular joint, periodontal ligament and

mucosa detect the presence of a bolus and its mastication and

stimulate the salivary nuclei to increase the parasympathetic

secretomotor discharge.

Gustatory stimuli

Sour stimuli are more effective, followed by sweet, salt and

bitter. Most foods also elicit olfactory stimuli and a reflex response

to smell can be demonstrated.

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Other stimuli

a) There appears to be connections between

the salivary nuclei and the vomiting center in the medulla,

since copious reflex salivation as well as nausea frequently

occur just before vomiting, perhaps as an attempt to dilute or

neutralized the irritant which is giving rise to nausea.

b) Hypersalivation (ptyalism) is also

described in pregnancy, but the physiologic basis is unclear;

perhaps it stems from morning sickness or oesophageal

irritation following reflex of gastric contents due to raised

abdominal pressure in late pregnancy.

c) Excess salivation may be seen associated

with motor disturbances of orofacial musculature.

Physiologic factors that effect salivation

1) Agreeable taste stimuli result in profuse salivation whereas

distasteful stimuli can result in temporary cessation of

salivation.

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2) A smooth object inserted into the mouth will result in an

increase in salivation whereas a rough object will inhibit

salivation (the polished surfaces of a denture should be

smooth).

3) Dehydration – Saliva decreases.

4) As one ages, the saliva becomes ropy in consistency.

5) Emotions and other psychic effects exite the autonomic

nervous system and in turn the function of body organs are

altered.

Pathologic conditions that decrease salivation:

1) Senile atrophy of the salivary glands.

2) Irradiation therapy for head and neck tumors.

3) Diseases of the brain sten that directly depress salivary

nuclei and block salivation.

4) Some types of encephalitis including poliomyelitis.

5) Diabetes mellitus and / or insipidus.

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6) Diarrhea caused by bacteria / food.

7) Elevated temperature from acute infectious diseases.

Pathologic conditions that increase salivation:

1. Digestive tract irritants.

2. Painful afflictions of the oral cavity.

These may be from:

a. Vitamin deficiency.

b. Trauma from surgery.

c. Ill fitting denture.

d. Inadequate interocclusal distance when

dentures are made.

Composition of saliva:

Saliva is made up of approximately 99% of water. The

concentrations of dissolved solids (organic and inorganic) are

characterized by wide variation.

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Protein

Comprise approximately 200mg/100ml. It includes enzymes,

immunoglobulins, other antibacterial factors, mucous glycoproteins

(mucins), traces of albumin and certain polypeptides and

oligopeptides.

-amylase

It is present in parotid saliva in concentrations of 60-

120mg/100ml and in submandibular at approximately by

25mg/100ml.

The enzyme hydrolyses the 1:4 glycosidic bond between

glucose units in the polysaccharide chain of starch, hydrolysis takes

place anywhere along the chain and thus the end products of

amylase digestion are mainly maltose together with

oligosaccharides and some free glucose.

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Immunoglobulins

Secretory IgA is the predominant immunoglobulin at

approximately 20mg/100ml with IgG (1.5mg/100ml) and IgM

(0.2mg/100ml).

Antibacterial proteins

Lysozyme is an antibacterial enzyme which attacks

components of the cell wall of certain bacteria leading to lysis.

Lactoferrin is an iron binding protein which removes free

iron from saliva depleting the supply of iron needed for bacterial

growth.

Sialoperoxidase oxidizes salivary thiocyanate to

hypothiocyanate, a potent antibacterial substance, using hydrogen

peroxide produced by oral bacteria as an oxidant.

Glycoproteins:

The major groups of glycoproteins consist of two classes of

mucous glycoproteins (MG1 and MG2) found in submandibular and

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sublingual saliva and a group of proline rich glycoproteins found in

parotid saliva.

Other polysaccharides

Siatherin is a small phosphoprotein (12000 daltons) relatively

rich in tyrosine and proline which has the property of inhibiting

hydroxy apatite crystal growth. It also prevents the precipitation of

calcium phosphates from supersaturated solutions and may be

important as an inhibitor of calculus formation both in glands and

on the teeth.

Sialin a tetrapeptide can be utilized by several bacteria

leading to formation of alkaline end products which are believed to

help regulate pH of plaque.

Other organic compounds:

Free amino acids with concentration below 0.1/100ml.

Urea is present in levels of about 12-20mg/100ml. It is

hydrolysed by many bacteria with the release of ammonia leading

to rise in pH.

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Glucose concentration is about 0.5-1mg/100ml but may be

raised in diabetics.

Inorganic constituents:

1) Sodium (0-80mg/100ml).

2) Potassium (60-100mg/100ml).

3) Calcium (2-11mg/100ml).

4) Phosphorous (6-71mg /100ml).

5) Chloride (50-100mg/100ml).

6) Thiocyanate.

7) Fluoride (0.01-0.04mg/100ml).

8) Bicarbonate (0-40mg/100ml).

Factors influencing the composition of saliva

1) Flow rate:

As the flow rate increases, the concentrations of proteins,

sodium, chloride and bicarbonate rise, while the levels of

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phosphate and magnesium fall. This is true for direct saliva as

well as whole saliva.

2) Differential gland contributions

In unstimulated whole saliva, the parotid glands contribute

only about 10% of the fluid volume, whereas on stimulated

saliva they become predominant. Thus the composition of the

mixed fluid approaches that of parotid saliva at high flow rates.

As the calcium concentration in parotid saliva is lower than that

in submandibular saliva, stimulated whole saliva secretion,

despite the fact that the levels may rise in both direct salivas as

their flow rate increases.

3) Circadian rhythm:

Rhythmic variations in the concentrations of many salivary

constituents are seen. Example: The levels of calcium and

phosphorous ions are low in mornings.

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4) Duration of stimulus:

As a constant rate of flow, the composition may vary with the

duration of stimulation.

5) Nature of stimulus:

If the flow rate is held constant the secretions elicited with

sour, salt, sweet or bitter stimuli are similar in electrolyte

composition. However, salt stimulates a higher protein content.

Sugar stimuli give rise to a high amylase content. None of these

are biologically significant.

6) Diet:

Long term changes in diet do not appear to have a dramatic

effect on salivary composition. Changes in plasma phosphate

and urea concentrations induced by dietary alterations may be

reflected in the saliva.

Properties and Function of Saliva

1) Digestion:

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Salivary amylase initiates the digestion of starch but is

inactivated in the stomach because of low pH and proteolytic

acid.

However, considerable breakdown of starch can occur prior

to inactivation of salivary amylase and especially after a heavy

meal this may account for a significant fraction of starch

digestion.

2) Lubrication:

The lubrication of hard and soft oral surfaces is very

important for speech, mastication and swallowing and for

general oral health and comfort. Saliva provides a tissue coating

film which is responsible for lubrication and bolus formation, a

property which is due to water and mucous glycoproteins it

contains.

3) Dilution and clearance:

Another effect of water content of saliva is the dilution of

substances introduced into the mouth, and their subsequent

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removal by swallowing or spitting. Food solids are first

dissolved (whereby they can be tasted) and diluted as the saliva

flows into the mouth. After swallowing the bulk of food or

drink, the food residues are cleared slowly by the continuing

flow of unstimulated saliva. If the substance is noxious or

distasteful, cleansing may be achieved by spitting.

Neutralization and buffering:

Saliva is alkaline and is an effective buffer system. These

properties protect the oral tissues against acids from food or from

plaque. The principle constituent responsible for these properties

(especially in stimulated saliva) is bicarbonate. The salivary

neutralization and buffering effect can markedly reduce the

cariogenic potential of foods. Phosphate and proteins also provide

small amount of buffering effect.

Saturation:

Saliva is supersaturated with respect to tooth material. This is

responsible for the growth of hydroxyapatite crystals during the

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remineralization phase of carious process. Salivary calcium and

phosphate are source of minerals for calculus formation.

Antibacterial Effects

Immunoglobulins

Salivary antibodies are mainly of IgA class whose principle

action is to aggregate specific bacteria and prevent their adhesion

to oral hard and soft tissues. Bactericidal lysis due to complement

activation may occur in gingival crevice as a result of the effects of

IgG in crevicular fluid which may also opsonise bacteria to assist

phagocytosis by PMNLS.

Bacterial competition:

Because of large number of bacteria in saliva, competition

for source nutrients is free and pathogens fastidious may thus be

unable to survive.

Pellicle and plaque formation

Pellicle formation is a physico-chemical process involving

selective sequential adsorption of salivary glycoproteins to the

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tooth surface. Pellicle protects teeth against both chemical and

mechanical insult. In addition it acts as a substrate for colonization

by bacteria.

Plaque formation involves incorporation of salivary proteins

but their characterization is difficult because they are extensively

degraded in plaque.

PROSTHODONTIC IMPLICATIONS

The functions of saliva in edentulous subjects are:

1) Saliva removes debris and damaging factors by transportation

to oropharynx. Coats the soft tissues with film 0.1mm thick

(therefore to maximize physical retentive factors, any space

between complete dentures and supporting tissues should not

exceed this value). The rate of debris clearance depends on

velocity of movement of film (0.8-8.0mm/minute, depending

on salivary flow rate and environmental muscle activity.

Irritants within oral cavity (e.g. new dentures) simulate

secretion.

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2) Antimicrobial action most effective against non-commensal

organisms. It is thus a determinant of the bacterial

composition of denture plaque which itself may be associated

with denture related stomatitis.

3) Hydrophilic components lubricate, moisten and protect

moving tissues in contact aided by pellicle formation.

Especially important for denture wearers. Secretions of minor

salivary glands are critical, senile dentures open onto all

mucosal surfaces (except anterior region of hard palate).

Saliva plays an important role as a physical agent in the

retention of complete dentures.

The physical factors consists of :

a) Adhesion.

b) Cohesion.

c) Surface tension.

d) Capillary attraction.

e) Atmospheric pressure.

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a) Adhesion: It is the physical molecular attraction of

unlike surfaces in close contact.

It acts when saliva wets and sticks to the basal surfaces of

dentures and at the same time to the mucous membrane of the

basal seat.

Effectiveness of adhesion depends upon close adaptation of

denture base to the supporting tissues and fluidity of saliva.

b) Cohesion: It is the molecular attraction between two

similar surfaces in close contact.

It occurs in the layer of saliva between the denture base and

mucosa.

c) Interfacial surface tension:

It is the resistance to separation possessed by the film of

liquid between two well adapted surfaces.

It is found in the thin film of saliva between the denture base

and the mucosa of basal seat.

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d) Capillary attraction:

It is the force that causes the surface of liquid to become

elevated or depressed when it is in contact with a solid.

When the adaptation of denture base to mucosa on which its

rests is sufficiently close, the space filled with a thin film of

saliva acts like a capillary tube and helps retain the denture.

Saliva as a physiological factor of retention affects the

effectiveness of physical forces.

The higher the viscosity occurring to the mucoid content, the

lower the flow and greater is the fixation. Hence the mucos saliva

provides better cohesion that serous saliva.

Distribution of saliva

Saliva may be distributed in one of the three ways:

1) Saliva film covers all surface of the denture and the oral

mucous membrane. With this distribution of saliva there is no

meniscus and no contribution to retention by surface tension.

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2) The saliva film covers the oral mucous membrane but is

incomplete over the polished surface of the denture. This

distribution of saliva produces a meniscus and hence there is a

retentive force due to surface tension.

3) Saliva beneath the tissue basal surface of denture only. With

this distribution of saliva, there is meniscus and a considerable

retentive force due to surface tension.

Practical applications of knowledge of salivary glands and

saliva in complete denture prosthodontics:

1) Excessive salivation particularly by maxillary and sublingual

glands presents a problem in impression making.

When this problem exists, atropin sulfate can be administered

orally prior to impression making.

The new dentures may feel like foreign bodies and stimulate

the flow of saliva.

The patient should be assured that the feeling and flow of

saliva will decrease gradually.

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2) Excessive secretions of mucous from the palatal glands may

distort the impression material in the posterior two thirds of

the palate. To counteract this problem.

a) The palate may be massaged to encourage the glands

to empty.

b) The mouth may be irrigated with an astringent

mouthwash just prior to inserting impression material.

c) The palate may be wiped with gauge.

Failure to counteract the thick and ropy saliva because

of heavy secretions of mucous from palatal glands under

maxillary denture will result in:

i. Dislodgement of the denture.

ii. Presence of voids in the impression surface

while the impression material sets.

iii. Forms a factor in causing the patient to gag

while impressions are made and after

placement of new dentures.

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Ideally there should be a moderate flow of serous type saliva

which seems to be the situation most frequently found.

3) When it is determined that saliva is a problem, the cause

should be investigated. If necessary, therapy should be

instituted to correct the problem.

4) When the cause is undetermined or there is no favourable

response to therapy, the problem should be discussed with

the patient prior to treatment so that by patients

understanding, patients cooperation is assured.

5) A lack of saliva, xerostomia, presents some serious

problems:

a) The possibility of reduced retention increases.

b) Absence of saliva often causes cheeks and lips to stick

the denture base in uncomfortable manner.

Petroleum jelly applied over the surface can alleviate

this problem.

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c) Intolerance to denture wear. It may be necessary to

limit denture use to short periods.

d) Problems in mastication, swallowing, speaking etc.

this predisposes to nutritional deficiency.

The diet should be restricted to nutritious moist foods that

are soft or liquid.

True xerostomia occurs with a resting flow rate of 0.1ml/min.

or less.

Causes:

- Lack of or diseases of salivary glands (e.g. ectodermal

dysplasia, Sjogren’s syndrome).

- Irradiation of head and neck.

- Medication is a potent cause of xerostomia in the

elderly.

Common drug types producing xerostomia:1) Anti arrhythmics.

2) Anticonvulsants.

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3) Antidepressants.

4) Antihistamines.

5) Antinauseants.

6) Antineoplastics.

7) Antiparkisonian drugs.

8) Antipsychotics.

9) Antispasmodics.

10) Atropines.

11) Diuretics.

12) Hypotensive agents.

13) Minor tranquilizers.

14) Muscle relaxants.

15) Narcotics.

16) Sympathomimeteics.

Approximately one third of the adult population has

symptoms suggestive of dry mouth. It is important to appreciate

that the complain is a normal consequence of aging.

XEROSTOMIA

Salivary flow less than 0.2ml/minute.

Due to emotional reaction.

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Blockage of duct by calculus.

Acute and chronic infection of

salivary glands.

Administration of drugs.

Salivary gland aplasia.

Postoperative radiation damage

because of obliteration of major and minor salivary glands.

This is due to failure to adequately shield the salivary glands

which results in rapid atrophy and functional incapacity.

Vitamin A deficiency – A deficiency

of vitamin A affects specialized epithelium throughout the

body, including the epithelium of salivary glands.

It is also reported in patients with

riboflavin and nicotinic acid deficiencies.

Sjogrens syndrome (primary) – one

of the characteristic features of this disease (also called sicca

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syndrome) is xerostomia which occurs because of destruction

and atrophy of the acinar tissue of the salivary glands.

Pernicious anemia.

Loss of fluid from the body through

haemorrhage, excessive sweating, diarrhoea or vomiting.

Polyurea accompanying diabetes

mellitus and diabetes insipidus.

Organic lesions of nervous system

which interfere with normal secretory nerve stimulation.

CLINICAL FEATURES

Severe alterations in the mucous

membranes and the patient may have extreme discomfort.

The mucosa will appear dry and atrophic, sometimes

inflamed or more often pale and translucent.

The tongue may manifest the

deficiency by atrophy of the papillary inflammation,

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fissuring and cracking and in severe cases by areas of

denudation.

Soreness, hardening and pain of the

mucous membrane and tongue are common symptoms.

Patients complain of difficulty in

swallowing and talking, and also altered taste.

It will also predispose to

opportunistic oral infections, particularly candidasis and

leads to an increase in periodontal disease and caries.

Shallow but persistent tongue ulcers especially along the

posterior ventral surfaces are common in patients with xerostomia.

This is because of lack of protective lubrication as well as the

atrophic mucosa.

XEROSTOMIA PREDISPOSES TO:

Burning mouth syndrome (also

called as glossopyrosis, glossodynia or stomatopyrosis).

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It forms an etiological factor for

leukoplakia because of the lack of protective salivary layer.

Etiological factor for oral thrush

which along with chronic local irritants act by alteration of

oral mucous membranes predisposing then to invasion of

microorganisms.

Clinical examination:

In healthy patients saliva pooling in the floor of the mouth is

revealed whereas in xerostomia the amount of saliva is reduced but

may also appear frothy. A useful test is to place a mirror against the

buccal mucosa and this should lift off easily when saliva is present

in normal amounts. Degree of stickiness during this maneuver is a

useful indication of reduced saliva production. Each of the major

glands should be palpated and clear saliva seen to be expressed

from each duct orifice.

When Sjogren’s syndrome is

suspected a lower lip biopsy specimen should be taken, with

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five or six minor salivary lobules harvested from the

submusoa.

Assessment of antinuclear antibody

should be performed in patients with xerostomia, particularly

if Sjogren’s syndrome is suspected.

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MANAGEMENT

The treatment will depend upon the

nature of the disease and the cause should be detected and

corrected.

Adequate amount of saliva in the

mouth is achieved by use of artificial saliva substitutes, such

as saliva orthana, luborant and glandosane.

Xerolube is also a salivary

substitute.

Drug related xerostomia can be

minimized by lowering the dosage or changing the

medication in consultation with the patients physician.

Xerostomia associated burning is a

complex management problem since patients with Sjogren’s

syndrome or those who have undergone radiation therapy are

not able to regenerate salivary tissues. Salivary substitutes,

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transmucosal electro-stimulation and pilocarpine therapy may

all be of some benefit.

When all tests are negative, the

diagnosis is idiopathic xerostomia. Psychosomatic origin

must be considered and discussed with the patient. He or she

must be assured that no organic disease is present.

Chewing sugarless gum and sucking

sugarless candies stimulate flow of residual function.

Frequent sips of water and sucking

ice chips diminish the discomfort.

Aggressive use of oral hygiene

measures, fluoride and antibacterial agents are essential to

reduce the associated risks.

PROSTHODONTIC IMPLICATIONS

The mechanical protection of the

denture supporting tissues by the saliva film will be lost

predisposing then to irritation. This dryness of the mucosa

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renders it more susceptible to frictional irritation from

denture movement and may interfere with patients ability to

wear the dentures.

A decrease in salivary flow will

interfere with denture retention since the saliva contributes to

adhesion, cohesion, interfacial surface tension and

capillarity.

Difficulty in mastication and

deglutition.

The denture use should be limited to

short periods.

Restrict the diet to nutritious moist

food that are soft or liquid.

Good denture hygiene should be

encouraged since xerostomia predisposes to candidiasis.

REFERENCES:

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1) Blahova Zora et al: Physical factors in retention of complete

dentures. J Prosthet Dent 1971; 25: 230-235.

2) Boucher O. Carl : Bouchers prosthodontic treatment for

edentulous patients, ed. 9.

3) Edgar W.M. : Saliva : Its secretion, composition and

functions. British Dental Journal April 1992; 25: 305-312.

4) FDI working group 10, core, saliva : Its role in health and

disease. International Dental Journal 1992; 42: 291-304.

5) Heartwell M. Charles et al : Syllabus of complete dentures

ed. 4, Philadelphia 1992, Lea and Febiger.

6) Lindstrom R.E. et al : Physical – chemical aspects of denture

retention and stability : A review of literature. J. Prosthet

Dent 1979; 42: 371-375.

7) Mardel I.D. : The role of saliva in maintaining oral

haemostasis. JADA 1989; 119: 298-304.

8) Niedermeier H.W., Wilhelm et al: Salivary secretion and

denture retention. J Prosthet Dent 1992; 67: 211-216.

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9) Winkler Sheldon et al: Essentials of complete denture

prosthodontics, ed. 2, Delhi, 1996, A.I.T.B.S.

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